Many devices have been provided to harvest the thermal energy released by a hot source and to convert it into electricity.
Thermo-electromechanical transducers or thermoelectric generators formed from a tightly closed enclosure having one or a plurality of drops of a liquid enclosed therein have been provided in U.S. Pat. Nos. 8,773,003 and 8,378,558 and in French Patent No. 2986908 (all incorporated herein by reference). In such generators, the volume of the enclosure is much greater than that of the liquid that it contains, that is, the enclosure is mainly filled with gas or vapor of the liquid. When the liquid reaches a hot wall of the enclosure, a sudden evaporation creating an overpressure in the enclosure occurs. The overpressure causes a deformation of a membrane supporting a piezoelectric transducer, whereby electricity is generated. The generated vapor condensates into one or a plurality of drops on a cold wall of the enclosure and a new evaporation-condensation cycle can start.
A disadvantage of the thermoelectric generators of U.S. Pat. Nos. 8,773,003 and 8,378,558 is that they should be properly oriented so that the liquid drops displace from the cold wall to the hot wall under the effect of gravity.
The thermoelectric generator of French Patent No. 2986908 comprises means for promoting the displacement of a drop from the cold wall to the hot wall of an enclosure and for doing away with gravity. A disadvantage of this type of generator is that the displacement means are complex, whereby the generator is expensive to implement.
The inventors of the present application have provided in article “A microfluidic heat engine based on explosive evaporation”, Proceedings PowerMEMS 2012, pages 175-178 (incorporated herein by reference), a thermoelectric generator having a liquid introduced by a pump into a deformable chamber topped with a piezoelectric transducer. The chamber is heated so that a sudden evaporation of the liquid occurs, which causes an overpressure in the chamber. The overpressure causes a deformation of the chamber and of the piezoelectric transducer, and thus the generation of electricity. The generated vapor is then discharged into the atmosphere. A disadvantage of this type of generator is that it is necessary to use a pump, which makes the generator bulky.
Another disadvantage of the above-mentioned thermoelectric generators is that they have a pulsed operation and thus a low efficiency.
It would be desirable to have a thermal energy harvesting device which overcomes at least some of the disadvantages of existing generators.